Study on the femtosecond laser guided corona discharge

近年来，飞秒强激光脉冲在空气中形成的激光等离子体通道现象一直是当前科研领域的热点，飞秒光丝在很多领域都具有极其广泛的应用前景。由于飞秒激光具有很高的峰值功率，它在透明介质中传输时会产生光束的自聚焦，当由非线性克尔效应引起的自聚焦与等离子体散焦达到一种动态平衡时，就会形成很长的等离子体通道，称之为“飞秒成丝”现象。飞秒激光在传输介质中成丝之后，高强度激光脉冲可以实现远距离传输而不受瑞利距离限制，光丝通道内部含有大量的带电粒子。在成丝过程中伴随着诸多有趣的非线性效应，如：光强钳制、分子排列、高次谐波、自相位调制、锥角辐射等。正是由于这些有趣的物理现象，飞秒光丝被广泛的应用到超短脉冲压缩、白光激光雷达、遥感探测、THz辐射、人工降雨降雪、激光引雷等。因此，飞秒光丝的基本特性，尤其是光丝通道内等离子体密度空间分布对于飞秒光丝的应用具有重要意义。 本论文简要分析了飞秒激光在空气中成丝的物理过程以及光丝基本特性，并介绍了飞秒光丝在激光引雷中的应用以及飞秒激光等离子体通道的诊断方法。实验上开展了飞秒激光等离子体通道诱导电晕放电特性的初步研究，探究了几何构型对于飞秒光丝诱导电晕放电的影响，有利于理解光丝诱导高压的物理机制。此外，本文利用激光诱导电晕放电对低密度等离子体进行探测，并表征了飞秒光丝内等离子体密度的空间分布，同时对飞秒光丝的长度进行了精确测量，提出了一种探测等离子体密度空间分布的新方法。In recent years，laser filamentation produced by high peak power femtosecond pulse in air is one of the frontiers of high field laser physics. Femtosecond laser filament has shown broad applications in different fields. While propagating in optically transparent medium, high peak-power femtosecond pulses will collapse into a long plasma channel referred to as a“filament”due to the dynamic equilibrium between self-focusing due to Kerr effect and defocusing from self-generated plasma. Due to the filamentation, femtosecond pulse with high intensity can propagate for a long distance beyond the Rayleigh distance and the electron density inside the plasma channel is very high. Accompanying the generation of this long thin and relatively uniform ionization channel, some complex and fascinated nonlinear effects are observed, for example, intensity clamping, molecular alignment, harmonic generation, self-phase modulation, conical emission and so on. These nonlinear effects lead to many applications of filament in different fields, such as ultrashort pulse compression, LIDAR, remote sensing, terahertz emission, atmospheric water condensation and precipitation, lightning control. As a result, fully understanding the physical mechanism of the filamentation and filament characteristics are significantly important in filament applications. In this thesis, the physical mechanism of the filamentation and characteristics of the plasma channel were briefly introduced. The application of laser filament in lighting control and the methods to diagnose the plasma of filament were also presented. Some experiments on the filament guided corona discharge were carried out which illustrated the impact of geometric configuration. The results are useful to understand the physical mechanism of the filament guiding of high voltage. Besides, a new method based on laser induced corona discharges to probe weakly ionized plasma was demonstrated for the first time. The spatial distribution of plasma density inside the filament was characterized and the length of filament was more precisely measured by this technique. The results offer a new way to probe plasma density spatial distribution